refactor: extract NATS.Server.JetStream.Tests project

Move 225 JetStream-related test files from NATS.Server.Tests into a
dedicated NATS.Server.JetStream.Tests project. This includes root-level
JetStream*.cs files, storage test files (FileStore, MemStore,
StreamStoreContract), and the full JetStream/ subfolder tree (Api,
Cluster, Consumers, MirrorSource, Snapshots, Storage, Streams).

Updated all namespaces, added InternalsVisibleTo, registered in the
solution file, and added the JETSTREAM_INTEGRATION_MATRIX define.

tests/NATS.Server.JetStream.Tests/JetStreamClusterReloadTests.cs

@@ -0,0 +1,64 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+
+namespace NATS.Server.JetStream.Tests;
+
+public class JetStreamClusterReloadTests
+{
+    [Fact]
+    public async Task Reload_rejects_non_reloadable_jetstream_storage_change()
+    {
+        await using var fixture = await ConfigReloadFixture.StartJetStreamAsync();
+
+        var ex = await Should.ThrowAsync<InvalidOperationException>(() => fixture.ReloadAsync("jetstream { store_dir: '/new' }"));
+        ex.Message.ShouldContain("requires restart");
+    }
+}
+
+internal sealed class ConfigReloadFixture : IAsyncDisposable
+{
+    private readonly string _configPath;
+    private readonly NatsServer _server;
+
+    private ConfigReloadFixture(string configPath, NatsServer server)
+    {
+        _configPath = configPath;
+        _server = server;
+    }
+
+    public static Task<ConfigReloadFixture> StartJetStreamAsync()
+    {
+        var configPath = Path.Combine(Path.GetTempPath(), $"natsdotnet-reload-{Guid.NewGuid():N}.conf");
+        File.WriteAllText(configPath, "jetstream { store_dir: '/old' }");
+
+        var options = new NatsOptions
+        {
+            ConfigFile = configPath,
+            JetStream = new JetStreamOptions
+            {
+                StoreDir = "/old",
+                MaxMemoryStore = 1_024 * 1_024,
+                MaxFileStore = 10 * 1_024 * 1_024,
+            },
+        };
+
+        var server = new NatsServer(options, NullLoggerFactory.Instance);
+        return Task.FromResult(new ConfigReloadFixture(configPath, server));
+    }
+
+    public Task ReloadAsync(string configText)
+    {
+        File.WriteAllText(_configPath, configText);
+        _server.ReloadConfigOrThrow();
+        return Task.CompletedTask;
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _server.Dispose();
+        if (File.Exists(_configPath))
+            File.Delete(_configPath);
+
+        return ValueTask.CompletedTask;
+    }
+}